If your outgoing transaction has been pending for a long time there's a chance it was dropped by the network. You can check its status by looking it up on a block explorer (tap on the transaction in your history > View advanced transaction data > Show in Block Explorer). If the block explorer finds your transaction, that means it's still waiting to be confirmed and you'll need to wait longer. If the block explorer can't find your transaction (404 error, says it's missing, etc), that means it has been dropped. To resolve, tap and hold on your wallet on the asset screen and then tap Resync.